1. Field of the Invention
This invention relates to a thermosensitive recording material comprising a leuco dye and a specific color developing agent, capable of producing color images with high reliability when heat is applied thereto.
2. Discussion of Background
Thermosensitive recording materials are, in general, composed of a substrate such as a sheet of paper or synthetic paper, or a resinous film, and a thermosensitive coloring layer formed thereon, which develops color when heat is applied thereto. For the application of heat to the coloring layer, a thermal printer with a built-in thermal head is usually employed.
The recording method using the above thermosensitive recording material is advantageous over the recording methods of other types in that it requires neither an image developing process nor an image fixing process, so that images can be recorded by using a relatively simple device in a short time. Since the thermosensitive recording materials can be produced inexpensively, the thermosensitive recording method is also economically advatageous. For the above and other advantages, the thermosensitive recording method is applied to various objects such as copying apparatus, facsimiles, ticket-vending apparatus, label printers, and recorders.
However, no perfect thermosensitive recording materials are known so far. For instance, some of the recording materials yield a so-called "head-dust" which sticks to the thermal head. The head-dust gives rise to sticking phenomenon, and also brings about abrasion of thermal head. The clearness of the recorded images is thus degraded.
Moreover, it is now greatly demanded to enhance the resistance to water and oil to improve the reliability of images recorded on tickets and labels.
In order to improve the reliability of recorded images, various studies have been made on each of the component layers of the thermosensitive recording material such as a thermosensitive coloring layer, an undercoat layer and an overcoat layer; and various color developing agents have also been proposed, for example, diphenylthioacetic ester derivatives as disclosed in Japanese Laid-Open Patent Application 62-271789, bis(4-hydroxyphenyl)acetic alkyl ester derivatives as disclosed in Japanese Laid-Open Patent Application 62-273885, carboxybenzamide compounds as disclosed in Japanese Laid-Open Patent Application 62-282972, p-toluene sulfonylhydrazide as disclosed in Japanese Laid-Open Patent Application 62-294590, 2,4'-dihydroxydiphenylsulfone as disclosed in Japanese Laid-Open Patent Application 63-3991, salicylic acid derivatives as disclosed in Japanese Laid-Open Patent Applications 63-22682, 63-22683, 63-28691 and 63-30283, pulenyltetramethylene sulfonium hexafluorophosphate as disclosed in Japanese Laid-Open Patent Application 63-45087, 2,4-bis(4.beta.-p-hydroxybenzoyloxycarbonyl)benzene as disclosed in Japanese Laid-Open Patent Application 63-72590, 3-aryl-4-hydroxyphenylsulfone as disclosed in Japanese Laid-Open Patent Application 63-77779, 3-allyl-4-hydroxy-4'-methyldiphenylsulfone as disclosed in Japanese Laid-Open Patent Application 63-82778, carboxylates having two hydroxyphenyl groups as disclosed in Japanese Laid-Open Patent Application 63-84979, substituted benzyl fumarates as disclosed in Japanese Laid-Open Patent Application 63-102981, bis(substituted-hydroxyphenyl)acetate derivatives as disclosed in Japanese Laid-Open Patent Application 63-128985, and .beta.-p-methoxyphenoxy-ethyltolyloxy acetate as disclosed in Japanese Laid-Open Patent Application 63-132084.
Aside from the above proposals, attempts have been made, in which a specific auxiliary component is incorporated into a thermosensitive coloring layer or an undercoat layer, to improve the reliability of recorded images. For example, alkaline salts of an organic carboxylic acid or its anhydride are incorporated into a thermosensitive coloring layer as disclosed in Japanese Laid-Open Patent Application 58-11193, metal salts of salicylaldoxime are incorporated into a thermosensitive coloring layer as disclosed in Japanese Laid-Open Patent Application 58-140291, zinc chloride and/or magnesium chloride is incorporated into an undercoat layer as disclosed in Japanese Laid-Open Patent Application 59-115892, a diphenyl ether compound having a formula of (X).sub.m -Ph-O- Ph-(X).sub.n is incorporated into a thermosensitive coloring layer as disclosed in Japanese Laid-Open Patent Application 59-214689, zinc salts of an acrylic acid or .alpha.-acrylic acid are incorporated into a thermosensitive coloring layer as disclosed in Japanese Laid-Open Patent Application 60-34891, and bisphenol-S bis-substituted acetylester is incorporated into a thermosensitive coloring layer as disclosed in Japanese Laid-Open Patent Application 62-294589.
As described above, many proposals have been made until now, but none of them can fully attain the aimed object, that is, the improvement of reliability of recorded images.